Recovery of bis(diarylphenol) ligands during the production of isopulegol

ABSTRACT

The present invention relates to a method of working up an aluminium-containing reaction product from the production of isopulegol by cyclization of citronellal in the presence of complex compounds, comprising at least one ligand of the formula (I), 
                         
where Ar 1 , Ar 2 , Ar 3 , Ar 4  are chosen from C 6 -C 15 -aryl or C 2 -C 15 -hetero; R 1 , R 2 , R 3 , R 4  are chosen from H, C 1 -C 6 -alkyl, C 1 -C 6 -perfluoroalkyl, C 1 -C 6 -alkoxy, C 7 -C 12 -aralkyl, halogen, SiR 5b R 6b R 7b , C 6 -C 10 -aryl, NR 8b R 9b , SR 10b , NO 2 ; and where R 1  or R 2  and/or R 3  or R 4 , together with A, can form an aromatic or nonaromatic cycle; etc.;
 
in which a) the aluminum-containing reaction product is subjected to distillative separation, b) the isopulegol-depleted bottom product is brought into close contact with an aqueous base and c) the ligand of the formula (I) is separated off from the organic phase, preferably by crystallization.
 
     Moreover, the invention relates to a method of producing isopulegol, and to a method of producing menthol.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. §371)of PCT/EP2007/059149, filed Aug. 31, 2007, which claims benefit ofEuropean Application No. 06120013.5, filed Sep. 1, 2006.

DESCRIPTION

The present invention relates to a method for working up analuminum-containing reaction product from the production of isopulegolby cyclization of citronellal in the presence of complex compounds,comprising at least one ligand of the formula (I).

Moreover, the invention relates to a method for producing isopulegol,and to a method for producing menthol.

In terms of amount, menthol is the most important aroma chemicalworldwide. The demand for menthol continues to be covered largely byisolation from natural sources. In addition, however, there are alsosynthetic routes to menthol, sometimes in racemic form, sometimes in theform of the natural enantiomer L-menthol.

An important intermediate for producing racemic such as optically activementhol is isopulegol, which is usually produced by a cyclizing oxo-enereaction of citronellal in the presence of Lewis-acidic catalysts and isusually produced in the form of mixtures of the four diastereomersisopulegol, iso-isopulegol, neo-isopulegol and neoiso-isopulegol.

Suitable catalysts which have been described for carrying out theabovementioned cyclization of citronellal to isopulegol are bothheterogeneous catalysts, such as SiO₂, Al₂O₃/SiO₂, SiO₂/ZrO₂, SiO₂/TiO₂mixed catalysts, moredenites, faujasites, montmorillonites andzeolites—and also homogeneous catalysts, such as, for example, sulfonicacids or Lewis acids, such as, for example, SnCl₄, ZnCl₂ or ZnBr₂.

EP-A 1 225 163 describes the cyclization of citronellal to isopulegol inthe presence of tris(2,6-diphenylphenol) aluminum catalysts. This methodfor cyclizing citronellal to isopulegol uses catalyst complexes whichare expensive and can only be produced with complexity. After thedescribed method, to be carried out in a homogeneous phase, the catalystcomplex is hydrolyzed when the reaction is complete. Possible recoveryand reusability of the ligand liberated in the process is not described.

By contrast, the unpublished PCT/EP 2006/060416 describesbis(diarylphenoxy)aluminum compounds which are obtainable by reacting abis(diarylphenol) ligand of the formula (I) with a suitable aluminumcompound, and methods of producing isopulegol and menthol in thepresence of these compounds. Here, processes are also disclosed whichpermit recovery of the bis(diarylphenol) ligands of the formula (I)used. Recovery takes place by crystallization from a bottom productobtained during the distillative separation of isopulegol from areaction product of the cyclization of citronellal. However, such awork-up leads to yields and purities which are not entirelysatisfactory, especially in the case of a continuous method forproducing isopulegol.

Accordingly, it was an object of the present invention to provide amethod which, after cyclization of citronellal to isopulegol has takenplace, permits the bis(diarylphenol) ligands used to be recovered andreused with improved purity and yield. Specifically, it was the aim tofacilitate a continuous method with good space-time yield.

Surprisingly, it has now been found that the bis(diarylphenol) ligandsused can be recovered in improved purity and yield after cyclization ofcitronellal by distillative separation of the aluminum-containingreaction product to give an isopulegol-enriched top product and anisopulegol-depleted bottom product, then bringing theisopulegol-depleted bottom product into contact with an aqueous base togive an aluminum-containing aqueous phase and an organic phasecomprising the majority of the ligands of the formula (I) and thenseparating off the ligand of the formula (I) from the organic phase,preferably by crystallization.

The present invention thus provides a method for working up analuminum-containing reaction product from the production of isopulegolby cyclizing citronellal, comprising

i) isopulegol,

ii) at least one ligand of the formula (I),

-   -   where    -   Ar¹, Ar², Ar³, Ar⁴, independently of one another, are chosen        from C₆-C₁₅-aryl radicals or C₂-C₁₅-heteroaryl radicals, which,        if appropriate, can in each case carry 1 to 7 identical or        different substituents chosen from C₁-C₆-alkyl,        C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen,        SiR^(5a)R^(6a)R^(7a), optionally substituted C₆-C₁₀-aryl,        NR^(8a), R^(9a), SR^(10a), NO₂,    -   R¹, R², R³, R⁴, independently of one another, are chosen from        hydrogen, C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy,        C₇-C₁₂-aralkyl, halogen, SiR^(5b)R^(6b)R^(7b), optionally        substituted C₆-C₁₀-aryl, NR^(8b)R^(9b), SR^(10b), NO₂ and where        -   R¹ or R² and/or R³ or R⁴, together with A, can form an            aromatic or nonaromatic cycle, and    -   A is a straight-chain or branched and/or cyclic hydrocarbon        radical having 1 to 25 carbon atoms which may be saturated or        mono- or polyunsaturated and/or partially aromatic and can, if        appropriate, have one or more identical or different heteroatoms        chosen from O, S, NR¹¹, and/or one or more identical or        different functional groups chosen from the functional groups        C(O), S(O), S(O)₂ and can, if appropriate, carry one or more        identical or different substituents chosen from the substituents        C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy,        C₇-C₁₂-aralkyl, halogen, —SiR^(5c)R^(6c)R^(7c), optionally        substituted C₆-C₁₀-aryl, substituted or unsubstituted        C₂-C₁₀-hetaryl, NR^(8c)R^(9c), SR^(10c), NO₂, C₁-C₁₂-acyl,        C₁-C₁₀-carboxyl, or        -   is a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical            which can, if appropriate, in each case carry 1 to 5            substituents chosen from C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,            C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5d)R^(6d)R^(7d),            substituted or unsubstituted C₆-C₁₀-aryl, NR^(8d), R^(9d),            SR^(10d), NO₂, or        -   is a functional group or a heteroatom chosen from the group            —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—,            —(R¹¹)P(O)— and —Si(R¹²R¹³),        -   where the radicals R^(5a), R^(6a), R^(7a), R^(8a), R^(9a),            R^(10a) to R^(5d), R^(6d), R^(7d), R^(8d), R^(9d), R^(10d),            R¹¹, R¹² and R¹³ are in each case independently of one            another chosen from C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or            substituted or unsubstituted C₆-C₁₀-aryl and where the            radicals R^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and            R^(9c), R^(8d) and R^(9d), independently of one another, can            in each case together also form a cyclic hydrocarbon radical            having 2 to 8 carbon atoms which can have one or more            identical or different heteroatoms chosen from the group O,            S, NR^(11a), and R^(11a) can have the meanings given for            R¹¹,        -   in free and/or complex-bound form,            in which

-   a) the aluminum-containing reaction product is subjected to    distillative separation to obtain an isopulegol-enriched top product    and an isopulegol-depleted bottom product,

-   b) the isopulegol-depleted bottom product is brought into close    contact with an aqueous base to give an aluminum-containing aqueous    phase and an organic phase comprising the majority of the ligands of    the formula (I),

-   c) the ligands of the formula (I) are separated off from the organic    phase.

In one preferred embodiment of the method according to the invention,the ligand of the formula (I) is separated off from the organic phase bycrystallization.

The bis(diarylphenol) ligands of the formula (I) obtained by the methodaccording to the invention can usually be converted to the reactivecatalyst complex without further purification steps, within the scope ofa new batch with the corresponding aluminum compounds of the formulae(II) or (III), as defined below, with no or no noteworthy decrease inthe reactivity being established with catalyst complexes recovered inthis way.

The bis(diarylphenol) ligands of the formula (I) have two phenol systemswhich in each case are substituted in both ortho positions relative tothe phenolic hydroxy group by aromatics or heteroaromatics (Ar¹ to Ar⁴)and are joined together via a structural element A and can, ifappropriate, also carry further substituents (R¹ to R⁴).

The aromatic or heteroaromatic substituents Ar¹ to Ar⁴ may,independently of one another, be identical or different. Preferably, thetwo substituents bonded in each case to a phenol system (Ar¹ and Ar² orAr³ and Ar⁴) are pairwise identical. Particularly preferably, all foursubstituents Ar¹ to Ar⁴ are identical.

The specified substituents Ar¹ to Ar⁴ are aryl radicals having 6 to 15,preferably 6 to 10, carbon atoms or heteroaryl radicals having 2 to 15,preferably 3 to 10, carbon atoms in the aromatic ring system. Arylradicals having 6 to 15 carbon atoms are, for example, phenyl, naphthyl,anthracenyl, preferably phenyl and naphthyl.

The specified heteroaryl radicals having 2 to 15 carbon atoms have 1 toabout 6, generally 1 to 3, identical or different heteroatoms which arechosen from the group of heteroatoms O, S and N. Examples thereof whichmay be mentioned are the following heteroaryl radicals: 2-furyl,3-furyl, 2-thienyl, 3-thienyl, 2-pyrrolyl, 3-pyrrolyl, 3-isoxazolyl,4-isoxazolyl, 5-isoxazolyl, 3-isothiazolyl, 4-isothiazolyl,5-isothiazolyl, 3-pyrazolyl, 4-pyrazolyl, 5-pyrazolyl, 2-oxazolyl,4-oxazolyl, 5-oxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl,2-imidazolyl, 4-imidazolyl, 1,2,4-oxadiazol-3-yl, 1,2,4-oxadiazol-5-yl,1,2,4-thiadiazol-3-yl, 1,2,4-thiadiazol-5-yl, 1,2,4-triazol-3-yl,1,3,4-oxadiazol-2-yl, 1,3,4-thiadiazol-2-yl and 1,3,4-triazol-2-yl,2-pyridinyl, 3-pyridinyl, 4-pyridinyl, 3-pyridazinyl, 4-pyridazinyl,2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 2-pyrazinyl,1,3,5-triazin-2-yl and 1,2,4-triazin-3-yl, 2-furyl, 3-furyl, 2-thienyl,3-thienyl, 2-pyrrolyl, 3-pyrrolyl, 3-isoxazolyl, 4-isoxazolyl,5-isoxazolyl, 3-isothiazolyl, 4-isothiazolyl, 5-isothiazolyl,3-pyrazolyl, 4-pyrazolyl, 5-pyrazolyl, 2-oxazolyl, 4-oxazolyl,5-oxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl, 2-imidazolyl,4-imidazolyl, 1,2,4-oxadiazol-3-yl, 1,2,4-oxadiazol-5-yl,1,2,4-thiadiazol-3-yl, 1,2,4-thiadiazol-5-yl, 1,2,4-triazol-3-yl,1,3,4-oxadiazol-2-yl, 1,3,4-thiadiazol-2-yl and 1,3,4-triazol-2-yl,2-pyridinyl, 3-pyridinyl, 4-pyridinyl, 3-pyridazinyl, 4-pyridazinyl,2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 2-pyrazinyl,1,3,5-triazin-2-yl and 1,2,4-triazin-3-yl, benzofuryl, isobenzofuryl,benzothienyl, indolyl, isoindolyl, carbazolyl, pyridyl, quinolyl,isochinolyl and pyrazyl. Preferred heteroaryl radicals are, for example:2-furyl, 2-pyridyl, 2-imidazoyl.

The aryl or heteroaryl radicals specified above for Ar¹ to Ar⁴ can, ineach case independently of one another, be unsubstituted or carry 1 toabout 7, preferably 1 to 3, in particular 1 or 2, identical or differentsubstituents which are chosen from the group of substituents:C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl,halogen, —SiR^(5a)R^(6a)R^(7a), substituted or unsubstitutedC₆-C₁₀-aryl, —NR^(8a)R^(9a), —SR^(10a), —NO₂, where the radicals R^(5a),R^(6a), R^(7a), R^(8a), R^(9a), R^(10a) and R¹¹ to R¹³, in each caseindependently of one another, are C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/orsubstituted or unsubstituted C₆-C₁₀-aryl, and the radicals R^(8a) andR^(9a), independently of one another can in each case together also forma cyclic hydrocarbon radical having 2 to 8 carbon atoms which can haveone or more identical or different heteroatoms chosen from the group O,S and NR^(11a), and R^(11a) can have the meanings given for R¹¹.

In this connection, the specified substituents within the scope of theoverall present invention have the meanings given below by way ofexample:

C₁-C₆-alkyl such as, for example, methyl, ethyl, propyl, 1-methylethyl,butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl,cyclopentyl, 1-methylbutyl, 2-methylbutyl, 3 methyl butyl,2,2-dimethylpropyl, 1-ethylpropyl, hexyl, cyclohexyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl,3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl,1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl,3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl,1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl and1-ethyl-2-methylpropyl;C₁-C₆-perfluoroalkyl, such as, for example, trifluoromethyl,pentafluoroethyl, heptafluoropropyl, heptafluoroisopropyl,nonafluorobutyl;C₁-C₆-alkoxy, such as, for example, methoxy, ethoxy, propoxy,1-methylethoxy, butoxy, 1-methylpropoxy, 2-methylpropoxy and1,1-dimethylethoxy, pentoxy, 1-methylbutoxy, 2-methylbutoxy,3-methoxylbutoxy, 1,1-dimethylpropoxy, 1,2-dimethylpropoxy,2,2-dimethylpropoxy, 1-ethylpropoxy, hexoxy, 1-methylpentoxy,2-methylpentoxy, 3-methylpentoxy, 4-methylpentoxy, 1,1-dimethylbutoxy,1,2-dimethylbutoxy, 1,3-dimethylbutoxy, 2,2-dimethylbutoxy,2,3-dimethylbutoxy, 3,3-dimethylbutoxy, 1-ethylbutoxy, 2-ethylbutoxy,1,1,2-trimethylpropoxy, 1,2,2-trimethylpropoxy, 1-ethyl-1-methylpropoxyand 1-ethyl-2-methylpropoxy,C₇-C₁₂-aralkyl, such as, for example, benzyl, 1-phenylethyl,2-phenylethyl;C₁-C₁₀-acyloxy, such as, for example, acetyloxy, propionyloxy;C₁-C₁₀-carboxyl, such as, for example, methoxycarbonyl, ethoxycarbonyl,propyloxycarbonyl, isopropyloxycarbonyl;C₁-C₁₀-acyl, such as, for example, formyl, acetyl, propionyl.

The expression “substituted or unsubstituted C₆-C₁₀-aryl” within themeaning of the present invention is aryl radicals which, as specifiedabove, have one or more, generally 1 to about 3, identical or differentsubstituents, where the substituents may be chosen, for example, fromC₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl,halogen, silyl, dialkylamino and nitro.

Within the scope of the present invention, the term “halogen” isfluorine, chlorine, bromine and iodine, preferably fluorine andchlorine.

Within the scope of the present invention, the substituents—SiR^(5a)R^(6a)R^(7a) to —SiR^(5d)R^(6d)R^(7d) are in each caseunderstood as meaning silyl substituents each having, independently ofone another, three identical or different radicals which are chosen fromthe radicals C₁-C₆-alkyl, C₇-C₁₂-aralkyl and substituted orunsubstituted C₆-C₁₀-aryl. By way of example, mention may be made here,for example, of the silyl substituents trimethylsilyl, triethylsilyl,tert-butyldimethylsilyl and tert-butyldiphenylsilyl.

Within the scope of the present invention, the substituents—NR^(8a)R^(9a) to —NR^(8d)R^(9d) are in each case amino substituentswhich, in each case independently of one another, carry two identical ordifferent, preferably two identical, radicals which are chosen from theabovedescribed radicals C₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substitutedor unsubstituted C₆-C₁₀-aryl. By way of example, mention may be made ofthe amino substituents: dimethylamino, diethylamino, dibenzylamino,diallylamino, diisopropylamino. Within the scope of the presentinvention, the radicals R^(8a) and R^(9a) to R^(8d) and R^(9d) mayindependently of one another, in each case together also form a cyclichydrocarbon radical having 2 to 8 carbon atoms which can have one ormore identical or different heteroatoms chosen from the group O, S,NR^(11a). The radical R^(11a) can here be abovedescribed C₁-C₆-alkyl,C₇-C₁₂-aralkyl and/or substituted or unsubstituted C₆-C₁₀-aryl. Examplesof these cyclic substituents R^(8a) and R^(9a) to R^(8d) and R^(9d)which may be mentioned are: piperidinyl, morpholinyl,N-methylpiperazinyl, N-benzylpiperazinyl.

In the substituents —SR^(10a), the radical R^(10a) is an abovedefinedC₁-C₆-alkyl, C₇-C₁₂-aralkyl and/or substituted or unsubstitutedC₆-C₁₀-aryl, preferably methyl, ethyl, isopropyl, phenyl, benzyl.

Within the scope of the present invention, preferred aromatic orheteroaromatic substituents Ar¹, Ar², Ar³, Ar⁴ are, for example, phenyl,4-methylphenyl, 2,4,6-trimethylphenyl, naphthyl, 2-fluorophenyl,4-fluorophenyl, 4-chlorophenyl, 4-bromophenyl, 3-fluorophenyl,3-chlorophenyl, 3,5-difluorophenyl, 3,5-dichlorophenyl,2,3,6-trichlorophenyl, 2,4,6-trichlorophenyl, 2-methylphenyl,4-methylphenyl, 2,4,5-trimethylphenyl, 2,4,6-trimethylphenyl.2-isopropylphenyl, 4-isopropylphenyl, 4-tert-butylphenyl,4-n-butylphenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl,3,5-bis(trifluoromethyl)phenyl, 4-arylphenyl, 3-nitrophenyl, preferably4-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl,3-trifluoromethylphenyl, 4-trifluoromethylphenyl. Within the scope of apreferred embodiment, the radicals Ar¹, Ar², Ar³, Ar⁴ are identical andare preferably 4-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl,3,5-dichlorophenyl, 3-trifluoromethylphenyl, 4-trifluoromethylphenyl,particularly preferably phenyl.

According to the invention, the substituents R¹, R²R³, R⁴ in the meta orpara position relative to the respective phenolic hydroxy groups may beidentical or different, preferably identical, and, in each caseindependently of one another, are hydrogen and/or an abovementionedC₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl,halogen, —SiR^(5b)R^(6b), R^(7b), substituted or unsubstitutedC₆-C₁₀-aryl, —NR^(8b)R^(9b), —SR^(10b) and/or —NO₂.

Preferred radicals R¹, R², R³, R⁴ which may be mentioned are: methyl,ethyl, isopropyl, halogen, in particular fluorine and/or chlorine,trifluoromethyl, phenyl, methoxy, nitro. Preferably, the radicals R¹,R², R³, R⁴ are identical and are particularly preferably hydrogen.

The radicals R¹ or R² and/or R³ or R⁴ may, together with the structuralelement A, also form a cyclic aromatic or nonaromatic cycle. In thesecases, the bis(diarylphenol) ligands of the formula (I) to be usedaccording to the invention have a tricyclic basic structure, for examplean anthracene basic structure of the formula (X) or basic structures ofthe type (XI):

Further structural modifications of these tricyclic basic structures, ifappropriate also those which have heteroatoms in the basic structure,are known to the person skilled in the art and belong to the group ofbis(diarylphenol) ligands which can be used according to the invention.

The structural element A in formula (I) can be a straight-chain orbranched and/or cyclic hydrocarbon radical having 1 to 25 carbon atoms,which may be saturated or mono- or polyunsaturated, normally 1 to about6-fold unsaturated and/or may be partially aromatic. The specifiedhydrocarbon radicals can, if appropriate, have one or more, generally 1to 3, identical or different heteroatoms chosen from the group ofheteroatoms O, S and NR¹¹ and/or one or more identical or differentfunctional groups chosen from the group of functional groups C(O), S(O)and S(O)₂, and if appropriate carry one or more identical or differentsubstituents chosen from the group of the substituents C₁-C₆-alkylC₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl,halogen, —SiR^(5c)R^(6c)R^(7c), substituted or unsubstitutedC₆-C₁₀-aryl, substituted or unsubstituted C₂-C₁₀-hetaryl,—NR^(8c)R^(9c), —SR^(10C), —NO₂, C₁-C₁₂-acyl and C₁-C₁₀-carboxyl.

Preferably, the structural element A in formula (I) is a straight-chainor branched and/or cyclic hydrocarbon radical having 1 to 25, preferably1 to 15 and particularly preferably 1 to 10, carbon atoms, which may besaturated or mono- to triunsaturated and/or partially aromatic. Thepreferred hydrocarbon radicals can, if appropriate, have one or more,generally 1 to 3, identical or different heteroatoms chosen from thegroup of heteroatoms O, S and NR¹¹ and/or one or more C(O) groups and,if appropriate, carry one or more identical or different substituentschosen from the group of substituents C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl, halogen, substituted orunsubstituted C₆-C₁₀-aryl, —NO₂, C₁-C₁₂-acyl and C₁-C₁₀-carboxyl.

Examples of structural elements A in the formula (I) which may bementioned without any limiting character are the following structuralelements 1 to 44, where the wavy lines in each case mark, as within thescope of the overall present disclosure, the linkage sites to theremainder of the particular ligand structure:

The structural elements 1 to 44 shown can in each case also carry thesubstituents referred to above and, if appropriate, have further,usually 1 or 2, ethylenic double bonds.

The structural element A can also be an aryl radical having 6 to 15,preferably 6 to 10, carbon atoms, specifically a phenylene, naphthyleneor anthracenylene radical, or a heteroaryl radical as defined abovehaving 2 to 15, preferably 3 to 10, carbon atoms.

The specified aryl and heteroaryl radicals may, if appropriate, in eachcase carry 1 to 5 substituents which are chosen from the group ofabovedescribed substituents C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,C₁-C₆-alkoxy, C₇-C₂-aralkyl, halogen, —SiR^(5d)R^(6d), R^(7d),substituted or unsubstituted C₆-C₁₀-aryl, —NR^(8d), R^(9d), SR¹⁰ andNO₂.

Furthermore, the structural element A can also be a functional group ora heteroatom which is chosen from the group —O—, —S—, —N(R¹¹)—, —S(O)—,—C(O)—, —S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—, —OP(O)O—, —OP(O)₂O— and—Si(R¹²)(R¹³)—, where the radicals R¹¹, R¹², R¹³, independently of oneanother, are in each case an abovedescribed C₁-C₆-alkyl, C₇-C₁₂-aralkyland/or substituted or unsubstituted C₆-C₁₀-aryl. Within this group, thestructural element A is preferably —O—, —S—, —S(O)—, —S(O)₂— or—Si(R¹²)(R¹³)—.

Within the scope of the present invention, the term “ligand in free orcomplex-bound form” comprises both the free form of the ligand and allconceivable forms which can be converted into the free form under theprocess conditions. Examples thereof which may be mentioned arealkoxides of the ligand, which are converted to the free form of theligand by basic hydrolysis.

Within the scope of the present invention, the expression “aqueous base”generally comprises aqueous solutions whose pH is greater than 7. Inparticular, these are aqueous solutions of alkali metal and alkalineearth metal hydroxides, specifically aqueous solutions of KOH and NaOH.

Within the scope of the present invention, the expression“aluminum-containing reaction product” is a reaction product whichcomprises at least one compound which comprises aluminum in ionic,covalent or complex-bound form. These are compounds of aluminum asresult under the conditions of the method according to the inventionfrom the compounds of the formula (R¹⁴)_(3-p)AlH_(p) (II) or MAlH₄(III), as defined below, used in the cyclization of citronellal. Withinthe scope of the present invention, the expression “majority” should beunderstood as meaning a percentage fraction of the total amount of acompound present which is greater than 50%, preferably greater than 80%and particularly preferably greater than 90%.

Step a):

In step a) of the method according to the invention, thealuminium-containing reaction product from the production of isopulegolby cyclization of citronellal is subjected to distillative separation togive an isopulegol-enriched top product and an isopulegol-depletedbottom product.

In a specific embodiment, step a) uses a solvent with a higher boilingpoint than that of the isopulegol. In this way, undesired thermalstressing of the bottom contents can be avoided. In particular, theligands of the formula (I) present therein are not in a form free fromsolvent while separating off the isopulegol. The higher-boiling solventcan be added to the aluminum-containing reaction product before and/orduring distillative separation. Preference is given to using ahigh-boiling solvent whose boiling point under the conditions of thedistillation is above the boiling point of the isopulegol. Preferably,the boiling point of the introduced solvent under the conditions of thedistillation is at least 5° C., preferably at least 10° C. and inparticular at least 20° C., above the boiling point of the isopulegol.

Preferred higher-boiling solvents which have such a boiling point are,for example, hydrocarbons, such as phenylcyclohexane, benzyltoluene,dibenzyltoluene, 1-methyl-naphthalene and tridecane, 1-decanol,1,2-propylene carbonate, ethers, such as diethylene glycol dibutylether, tetraethylene glycol dimethyl ether and dibenzyl ether, andtechnical-grade mixtures of these solvents. Particular preference isgiven to mixtures which comprise phenylcyclohexane as main constituent.

When using at least one higher-boiling solvent, the isopulegol-depletedbottom product in step a) obtained is an organic phase comprising thehigher-boiling solvent, the majority of the ligands of the formula (I)and, if appropriate, at least one aluminum-containing compound.

Preferably, distillative separation of isopulegeol in step a) takesplace at a bottom temperature of preferably at most 250° C., preferablyat most 150° C. and particularly preferably at most 100° C. The lowerbottom temperature is usually uncritical and is generally at least 0°C., preferably at least 20° C. To maintain these maximum temperatures,the distillation can, if desired, be carried out under a suitablevacuum.

The pressure in step a) of the method according to the invention is,irrespective of the specific embodiment, generally in a range from 0.1to 1500 mbar, preferably in a range from 1 to 500 mbar and particularlypreferably in a range from 5 to 100 mbar.

Irrespective of the composition of the aluminum-containing reactionproduct from the cyclization of citronellal and of the use of ahigher-boiling solvent, distillative separation of the isopulegol cantake place continuously or batchwise, preferably continuously. In onesuitable procedure, the higher-boiling solvent is added to the reactionproduct from the cyclization of citronellal before distillativeseparation and in the course of the distillation the amount ofhigh-boiling solvent present in the bottom is subsequently keptconstant.

For the distillative separation in step a), the customary devices knownto the person skilled in the art can be used (see e.g. Sattler,Thermische Trennverfahren [Thermal separation methods], 2nd Edition1995, Weinheim, p. 135ff; Perry's Chemical Engineers Handbook, 7thEdition 1997, New York, Section 13). These include distillation columnswhich may be provided with packings, internals etc. The distillationcolumns used can comprise separation-effective internals, such asseparation trays, e.g. perforated trays, bubble-cap trays or valvetrays, arranged packings, e.g. sheet-metal or fabric packings, or randombeds of packings. The number of plates required in the column(s) usedand the reflex ratio are essentially governed by the purity requirementsand the relative boiling position of the constituents in thealuminum-containing reaction product from the production of isopulegolby cyclization of citronellal and of the higher-boiling solvent, wherethe person skilled in the art can ascertain the specific design andoperating data by known methods. The distillative separation can, forexample, take place in one or more distillation columns coupledtogether.

Likewise suitable for the distillative separation in step a) arecustomary evaporators, preferably evaporators with forced circulation,particularly preferably falling-film evaporators.

Depending on additional components which may, if appropriate, be presentin the aluminum-containing reaction product from the cyclization ofcitronellal, the composition of the top product obtained duringdistillative separation may make it necessary to subject said product,if appropriate, to a further work-up step.

In a specific embodiment of the method according to the invention forworking up an aluminum-containing reaction product from the productionof isopulegol by cyclizing citronellal, the reaction productadditionally comprises a lower-boiling solvent (iii).

Within the scope of the present invention, the expression “lower-boilingsolvent (iii)” refers to the boiling point of the isopulegol. Ofparticular suitability here are those solvents or solvent mixtureswhich, under the conditions of the distillative separation, have aboiling point which is at least 5° C., preferably 10° C. and inparticular 20° C. below that of the isopulegol under the respectiveconditions.

Within the scope of the present invention, preferred solvents with sucha boiling point are inert organic solvents or mixtures thereof, such as,for example, aromatic solvents, e.g. toluene, ethylbenzene or xylene,halogenated solvents, e.g. dichloromethane, dichloroethane orchlorobenzene, aliphatic solvents, e.g. pentane, hexane or cyclohexane,ethers, e.g. tetrahydrofuran, diethyl ether, methyl tert-butyl ether,esters, e.g. ethyl acetate, or dimethylformamide (DMF), dimethylsulfoxide (DMSO) and the like. Particular preference is given totoluene.

If the aluminum-containing reaction product to be worked up comprisessuch a low-boiling solvent, then this is removed at least partially fromthe reaction product in a suitable embodiment prior to the distillativeseparation of the isopulegol. The lower-boiling solvent is likewisepreferably separated off by distillation. Depending on the boiling pointof the lower-boiling solvent, the customary abovementioned distillationdevices can be used.

In a further suitable embodiment, distillative separation of thealuminum-containing reaction product in step a) takes place to give anisopulegol-enriched top product which at the same time comprises atleast some, preferably the majority, of the lower-boiling solvent. Inthis case, the top product can be subjected to further separation,preferably likewise by distillation.

The separated-off lower-boiling solvent is advantageously returned tothe cyclization of the citronellal by using it as solvent. In this way,the method according to the invention requires—apart from top-ups whichare required as a result of unavoidable losses—just the single provisionof an amount of the lower-boiling solvent.

In a specific embodiment of the method according to the invention forworking up an aluminum-containing reaction product from the productionof isopulegol by cyclizing citronellal, the reaction productadditionally comprises an auxiliary (iv).

Within the scope of the present invention, the term “auxiliary” (iv)refers to compounds which are added during the cyclization ofcitronellal in order to suppress undesired secondary reactions.Preferably, the auxiliaries (iv) are chosen from organic acids,carboxylic anhydrides, aldehydes, ketones and vinyl ethers.

Specifically, the auxiliaries (iv) are chosen from acids, preferablyorganic acids. By way of example, organic acids which may be mentionedare: acetic acid, propionic acid, benzoic acid, toluenesulfonic acid,methanesulfonic acid, preferably acetic acid.

In a further specific embodiment of the present invention, theauxiliaries (iv) are chosen from carboxylic anhydrides, aldehydes,ketones and vinyl ethers.

The auxiliaries (iv) of said classes of substance can in each case bepresent individually or in the form of mixtures in the reaction productto be worked up. Preferred mixtures are those which consist of compoundsof one class of substance. The reaction product particularly preferablycomprises a single auxiliary.

Preferably, the auxiliaries (iv) present in the reaction product fromthe cyclization of citronellal are likewise at least partially removedand as far as possible returned to the cyclization of citronellal.

If the auxiliaries (iv) under the conditions of the distillation have aboiling point which is below or only slightly, i.e. less than 30° C.,above the boiling point of the isopulegol, these can be largelyrecovered from the fully reacted mixture by distillation to the extentto which it was not, if appropriate, itself reacted. Depending on theboiling point of the auxiliary, the customary abovementioneddistillation devices can be used.

If the auxiliaries (iv) have a boiling point under the conditions of thedistillation which is significantly above, i.e. at least 30° C., abovethe boiling point of the isopulegol, these remain in the bottom productand are, if appropriate, removed in step b) of the method according tothe invention if their physical properties allow this.

In a further suitable embodiment, distillative separation of thereaction product in step a) takes place to give an isopulegol-enrichedtop product which at the same time comprises at least some, preferablythe majority, of the auxiliary (iv). If appropriate, this main productcan comprise a lower-boiling solvent, as explained above. In this case,the top product can be subjected to further separation, preferablylikewise by distillation. The separated-off auxiliary (iv) is, ifappropriate together with the lower-boiling solvent, advantageouslyreturned to the cyclization of the citronellal, where it is used, forexample, for suppressing undesired secondary reactions. In this way, themethod according to the invention requires—apart from top-ups which arerequired as a result of unavoidable losses—just a single provision of anamount of the auxiliary (iv).

The separating off of isopulegol, the introduction of the higher-boilingsolvent and, if appropriate, the separating off of low-boilingcomponents, i.e. the separating off of any solvents present and volatileauxiliaries from the cyclization of citronellal, can be combined invarious ways:

In one suitable embodiment, a so-called dividing wall column is used forthe distillation, i.e. feed point and a side take-off are located onopposite sides of a dividing wall which extends along a section of thelongitudinal expansion of the column. Such distillation columns whichcomprise a dividing wall are known per se to the person skilled in theart. If side take-off and feed are in the region of the dividing wall, aconnection analogous to a Brugma or Petlyuk connection arises. Suchdistillations using dividing wall columns are described in DE-A-33 02525 and EP-A-0 804 951, to the entire scope of which reference is herebymade. In this case, a fraction enriched with low-boiling components canbe removed as top product, and a stream comprising the majority ofisopulegol can be removed as side take-off, for example. Thehigher-boiling solvent is introduced below the feed point, preferablyinto the bottom of the column or just above the bottom. A solution ofthe majority of the ligand of the formula (I) in the higher-boilingsolvent is produced as bottom product.

In an alternative embodiment, coupled columns are used for thedistillation. This embodiment may be advantageous if the reactionproduct of the cyclization of citronellal comprises a solvent and/or avolatile auxiliary, as explained in more detail below.

In this case, mixtures of isopulegol and lower- or slightlyhigher-boiling solvents and/or auxiliary (iv) can form the top productof the first column and in the second column be subjected to separationto give a stream comprising at least the majority of the isopulegol andan isopulegol-depleted stream comprising the lower-boiling solventsand/or auxiliaries of the cyclization.

Streams which can comprise lower-boiling solvents (iii) and auxiliary(iv) of the cyclization can usually be returned to the cyclizationwithout further separation.

The ligands of the formula (I) are produced, if appropriate in the formof their complexes or other derivatives, as bottom product of the firstcolumn.

Step b):

In step b) of the method according to the invention, theisopulegol-depleted bottom product is brought into close contact with anaqueous base to give an aluminum-containing aqueous phase and an organicphase comprising the majority of the ligands of the formula (I).Preferred aqueous bases are given above.

Besides the ligand of the formula (I) in free or complex-bound form, theisopulegol-depleted bottom product obtained in step a) can comprise atleast one further low-volatile component. These include, for example,higher-boiling solvents added in step a), the reaction products of thealuminum-containing compounds used for the cyclization of citronellal toisopulegol, and, if appropriate, auxiliaries (iv) not separated off instep a). Since aluminum-containing components and/or the auxiliaries(iv) accumulate particularly in the case of a continuous method and havean adverse effect especially on the yield and purity of the separationstep c), it is advantageous to remove these compounds as completely aspossible. This applies specifically to the aluminum-containingcompounds.

The bringing into contact in step b) preferably takes place byextraction. The number of extraction stages is preferably in a rangefrom 1 to 20 stages.

The extractants used are the abovementioned aqueous bases. Theseexpressions are therefore used synonymously within the scope of thepresent invention.

For the extraction, the isopulegol-depleted bottom product from step a)is brought into close contact with an aqueous base. Separation of thephases gives a phase comprising the majority of the ligand of theformula (I) and an aqueous phase enriched in aluminum-containingcompounds. The aqueous phase is then removed. The bringing into contactcan take place continuously or batchwise.

For the batchwise procedure the isopulegol-depleted bottom product fromstep a) and the aqueous extractant are brought into contact withmechanical agitation, e.g. by stirring, in a suitable vessel, themixture is left to stand for phase separation and one of the phases isremoved by expediently removing the denser phase at the bottom of thevessel.

A plurality of batchwise separation operations can be carried outsuccessively in a cascade-like manner, in which case the phase separatedoff from the aqueous phase and comprising the majority of the ligand ofthe formula (I) is in each case brought into contact with a freshportion of the aqueous extractant and/or the aqueous extractant ispassed countercurrently.

The extraction preferably takes place continuously. For the continuousextraction procedure, the aqueous extractant and the stream ofisopulegol-depleted bottom product from step a) is introducedcontinuously into suitable apparatuses in a manner analogous to thebatchwise variant. At the same time, a discharge of the phase comprisingthe majority of the ligand of the formula (I) and a discharge of theaqueous phase enriched in aluminum-containing compounds are continuouslyremoved from the apparatus in which the separation of the phases takesplace.

The extraction takes place at least in one stage, e.g. in amixer-separator combination. Suitable mixers are either dynamic orstatic mixers. Extraction in a plurality of stages takes place, forexample, in a plurality of mixer-separators or extraction columns.

In one suitable embodiment, at least one coalescing device is used toimprove phase separation. This is preferably chosen from coalescingfilters, electrocoalescers and combinations thereof. When usingmixer-separator devices for the extraction, the use of coalescingfilters, such as candle filters or sand filters, has proven advantageousfor improving phase separation. The filter can be installed heredirectly after the mixer (stirred container) and/or in the organicrun-off from the separator. Also preferred for improving phaseseparation is the use of electrocoalescers. These have proven useful forseparating off aqueous foreign phases of up to 5% by mass. The use ofcoalescing devices is also advantageously suitable in the methodaccording to the invention for separating off finely dispersed aqueousphase from the organic discharge of an extraction column comprising themajority of the ligand of the formula (I).

In one suitable embodiment, the extraction takes place in at least onemixer-separator combination for the extraction of aluminum-containingcomponents from the isopulegol-depleted bottom product from step a). Theuse of a further mixer-separator combination is particularlyadvantageous for subsequently reextracting and thus returning to theprocess fractions of the ligand of the formula (I) or, if appropriate,of the higher-boiling solvent which, if appropriate, with thealuminum-containing compounds to be separated off, partially pass intothe extractant.

Under certain circumstances, it may be advantageous to subject theorganic phase comprising the majority of ligands of the formula (I) to adrying step before separating off the ligand in step c) or afterseparating it off. Suitable drying methods are the customary ones knownto the person skilled in the art, in particular the adsorption todehydrating agents, e.g. using a zeolitic molecular sieve.

In an alternative embodiment of the method according to the invention,after bringing the isopulegol-depleted bottom product into contact withthe aqueous base, the water is completely or at least partially removedby distillation.

In order to prevent the ligand of the formula (I) from separating offprematurely, specifically by crystallization, at no point during step b)should the solubility product of the ligand in the organic phase beexceeded. This can be carried out through appropriate choice of thetemperature and/or the amount and type of added solvents, ifappropriate.

Consequently, in a preferred embodiment of the method according to theinvention, a discharge of the heated bottom product from step a) isbrought into close contact with a heated aqueous base.

Within the scope of the present invention, the expression “heated”refers to a temperature above room temperature and below the respectiveboiling point temperatures of the aqueous or organic solutions under thereaction conditions in question. In particular, heated refers to atemperature in the range from 25° C. to 150° C., specifically in therange from 70° C. to 100° C.

Depending on the auxiliaries used, if appropriate, in the cyclization ofcitronellal, the isopulegol-depleted bottom product can, if appropriate,comprise further components not separated off in step a). These arepreferably separated off in step b). In this case, the aqueous phaseobtained can be subjected to a suitable separation process in order torecover these components, e.g. an auxiliary (iv).

Step c).

In step c) of the method according to the invention, the ligand of theformula (I) is separated off from the organic phase comprising themajority of the ligand obtained in step b) by crystallization, wherestep c) can be carried out continuously or batchwise. Suitableembodiments of this step are, for example, crystallization and/orcomplete or at least partial distillative removal of volatileconstituents.

In one preferred embodiment of the method according to the invention,the ligand of the formula (I) is separated off by crystallization.

For the crystallization of the ligand of the formula (I), the solubilityproduct of the ligand of the formula (I) in the organic phase from stepb) must firstly be exceeded. This can take place, for example, by acooling process of the organic phase or by (partial) distillativeseparating off of the solvent. Methods for this purpose are known to theperson skilled in the art. For the technical configuration of the methodaccording to the invention, customary cooling crystallizers, evaporatingcrystallizers, vacuum crystallizers, crystallizing troughs or spraycrystallizers, for example, are suitable.

In one preferred embodiment of the method according to the invention,crystallization takes place by cooling the organic phase from step b) ofthe method. In general, crystallization takes place at a temperature inthe range from −50° C. to 100° C., preferably in the range from −20° C.to 50° C. and specifically in a range from 10° C. to 40° C.

This process can be accelerated by adding seed crystals.

The crystalline ligand of the formula (I) can be isolated from thesolution, for example, by filtration, flotation, centrifugation orsieving.

The ligand of the formula (I) retained in this way can, if appropriate,be dried by suitable drying methods. Methods for this are known to theperson skilled in the art. For example, for the technical configurationof the method, customary roller dryers, disk dryers, chamber dryers,fluidized-bed dryers or radiation dryers may be suitable.

The organic phase depleted in ligand of the formula (I) can again beadded to the process before or during step a).

In one suitable embodiment of the method according to the invention,crystallization takes place upon cooling to room temperature from aheated, saturated organic phase obtained in step b).

In a preferred embodiment of the method for working up a reactionproduct from the production of isopulegol, the ligand of the formula (I)is chosen from bis(diarylphenol) ligands of the formula (I.a)

where Ar¹, Ar², Ar³, Ar⁴, R¹, R², R³, R⁴ and A have the meanings givenabove.

The ligands of the formula (I.a) likewise have two phenol systems whichin each case are substituted in both ortho positions relative to thephenolic hydroxy group by aromatics or heteroaromatics (Ar¹ to Ar⁴) andare joined together via a structural element A and, if appropriate, canalso carry further substituents (R¹ to R⁴), the structural element Abeing joined to the two phenol systems in each case in the para positionrelative to the phenolic hydroxy group. Here, the radicals Ar¹, Ar²,Ar³, Ar⁴, the radicals R¹, R², R³, R⁴ and the structural element A canhave the same meanings as specified above for formula (I).

According to the invention, particularly preferred ligands are those inwhich the aryl radicals Ar¹, Ar², Ar³ and Ar⁴ are identical and have thepreferred meanings given above for formula (I). Particularly preferredaryl radicals Ar¹ to Ar⁴ are phenyl, naphthyl, 4-fluorophenyl,4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl, 4-methylphenyl,3-trifluoromethylphenyl, 4-trifluoromethylphenyl, very particularlypreferably phenyl.

In the ligands of the formula (I.a) preferred according to theinvention, the radicals R¹, R², R³, R⁴ are identical or different,preferably identical, and are preferably: hydrogen, halogen, inparticular fluorine or chlorine, methyl, trifluoromethyl, isopropyl,tert-butyl, phenyl, nitro.

The structural element A in formula (I.a) has the meanings given abovefor formula (I). Preferred structural elements A in formula (I.a) are inparticular also the structural elements 1 to 44 which may be substitutedin the specified manner.

Particularly preferred ligands are those of the formulae (I.a₁) to(I.a₃), where the specified radicals Ar¹ to Ar⁴, R¹ to R⁴ and R¹⁵ to R¹⁸preferably have the meanings listed by way of example in the table:

TABLE 1 (I.a₁)

Com- pound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁵ Ia₁-1 Ph Ph Ph Ph H H H H HIa₁-2 Ph Ph Ph Ph H H H H CH₃ Ia₁-3 Ph Ph Ph Ph H H H H Ph Ia₁-4 Ph PhPh Ph H H H H CF₃ Ia₁-5 Ph Ph Ph Ph H H H H CCl₃ Ia₁-6 Ph Ph Ph Ph H H HH 4-Cl-Ph Ia₁-7 Ph Ph Ph Ph H H H H CH₂CH₃ Ia₁-8 Ph Ph Ph Ph H H H H3-NO₂-Ph Ia₁-9 Ph Ph Ph Ph H H H H

TABLE 2 (I.a₂)

Compound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁶ R¹⁷ Ia₂-1 Ph Ph Ph Ph H H H HCF₃ CF₃ Ia₂-2 Ph Ph Ph Ph H H H H CCl₃ CCl₃ Ia₂-3 Ph Ph Ph Ph H H H HCH₃ CF₃ Ia₂-4 Ph Ph Ph Ph H H H H CH₃ CCl₃ Ia₂-5 Ph Ph Ph Ph H H H HCH₂CH₃ CF₃ Ia₂-6 Ph Ph Ph Ph H H H H CH₃ CH₃ Ia₂-7 Ph Ph Ph Ph H H H HCH₃ C(O)OCH₃ Ia₂-8 Ph Ph Ph Ph H H H H CH₃ C(O)OC₂H₅ Ia₂-9 Ph Ph Ph Ph HH H H —(CH₂)₃— Ia₂-10 Ph Ph Ph Ph H H H H —(CH₂)₄— Ia₂-11 Ph Ph Ph Ph HH H H —(CH₂)₅—

TABLE 3 (I.a₃)

Compound Ar¹ Ar² Ar³ Ar⁴ R¹ R² R³ R⁴ R¹⁸ Ia₃-1 Ph Ph Ph Ph H H H H—(CH₂)₂— Ia₃-2 Ph Ph Ph Ph H H H H

Ia₃-3 Ph Ph Ph Ph H H H H

Ia₃-4 Ph Ph Ph Ph H H H H

Ia₃-5 Ph Ph Ph Ph H H H H

Ia₃-6 Ph Ph Ph Ph H H H H

Ia₃-7 Ph Ph Ph Ph H H H H

Here, in Tables 1-3, Ph is a phenyl radical and C(O) is a carbonyl groupwithin the scope of the present invention. In general, the radicals R¹⁵,R¹⁶ and R¹⁷ can, independently of one another, be an abovedefinedC₁-C₆-alkyl, C₁-C₁₀-acyl, C₁-C₁₀-carboxyl or C₆-C₁₀-aryl, where thespecified radicals can carry one or more identical or different halogenand/or NO₂ substituents and where the radicals R¹⁶ and R¹⁷ can togetheralso form a cyclic structural element, preferably an alkylene bridge.

The present invention further provides a method for producing isopulegolof the formula (IV)

comprising

-   α) the cyclization of citronellal of the formula (V)

-   -   in the presence of a catalyst which is obtainable by reacting a        bis(diarylphenol) ligand of the formula (I) as defined in claims        1 and/or 10,    -   with an aluminum compound of the formula (II),        (R¹⁴)_(3-p), AlH_(p)  (II)    -   where    -   Al is aluminum,    -   R¹⁴ is a branched or unbranched alkyl radical having 1 to 5        carbon atoms and    -   p is 0 or an integer from 1 to 3,    -   and/or    -   with an aluminum compound of the formula (III),        MAlH₄  (III)    -   where    -   Al is aluminum and    -   M is lithium, sodium or potassium,

-   β) the recovery of the bis(diarylphenol) ligand of the formula (I)    after the reaction has taken place by    -   a) subjecting the aluminum-containing reaction product obtained        in step α) to distillative separation to obtain an        isopulegol-enriched top product and an isopulegol-depleted        bottom product,    -   b) bringing the isopulegol-depleted bottom product into close        contact with an aqueous base to give an aluminum-containing        aqueous phase and an organic phase comprising the majority of        the ligands of the formula (I) and    -   c) separating off the ligand of the formula (I) from the organic        phase.

In a specific embodiment of the method according to the invention forproducing isopulegol, the ligand of the formula (I) is separated off bycrystallization.

With regard to the preferred embodiments of the method according to theinvention for working up a reaction product from the production ofisopulegol by cyclization of citronellal, and for the preferred ligandsof the formula (I), reference is made to the abovementioned preferredembodiments in their entirety.

The bis(diarylphenol) ligands of the formulae (I) and (I.a) which can beused for producing the bis(diarylphenoxy)aluminum compounds usedaccording to the invention can be prepared easily by methods known perse to the person skilled in the art. Compounds of structure type (I.a₁)are obtained, for example, by reacting the correspondingbis-ortho-arylphenols with an aldehyde R¹⁵CHO in the presence of a Lewisacid, for example AlCl₃, as described, inter alia, by Z. Y. Wang, A. S.Hay in Synthesis 1989, 471-472 or in U.S. Pat. No. 3,739,035. Ligands ofstructure type (I.a₂) are, for example, accessible by reacting thecorresponding bis-ortho-arylphenols with a suitable ketone of theformula R¹⁶C(O)R¹⁷, as described, for example, in U.S. Pat. No.3,739,035. Ligands of structure type (I.a₃) are, for example, accessibleby Friedel-Crafts acylation of the corresponding phenols or O-protectedphenols with dicarboxylic acid chlorides, as described, for example, byF. F. Blicke et al. in J. Am. Chem. Soc. 1938, 60, 2283-2285; CH 350461or by G. Maier et al. in Chem. Bern 1985, 118, 704-721. Another way ofproducing ligands of structure type (Ia₃) also consists in theFriedel-Crafts alkylation of the corresponding phenols with tertiarydiols, as described, for example, in DE-A 25 34 558, or with dihalides,as described, for example, by J. Zavada, in Collect. Czech. Chem.Commun., 1976, 41, 1777-1790.

The bis(diarylphenoxy)aluminum compounds used according to the inventionare obtained, for example, by reacting the abovedescribedbis(diarylphenol) ligands of the formulae (I) or (I.a) with an aluminumcompound of the formula (II)(R¹⁴)_(3-p)AlH_(p)  (II).

Here, R¹⁴ is a branched or unbranched alkyl radical having 1 to 5 carbonatoms, such as, for example, methyl, ethyl, propyl, isopropyl, butyl,isobutyl, pentyl, isopentyl or neopentyl. The index p is 0 or an integerfrom 1 to 3. Preferably, the index p is 1 or 0, particularly preferably0. Preferred compounds of the formula (II) are, for example,trimethylaluminum, triethylaluminum, diisobutylalumninum hydride,particularly preferably trimethylaluminum and triethylaluminum.

Alternatively to this, the bis(diarylphenoxy)aluminum compounds usedaccording to the invention can also be obtained by reacting theabovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a)with an aluminum compound of the formula (III)MAlH₄  (III),where M is lithium, sodium or potassium. Consequently, of suitabilityfor producing the bis(diarylphenoxy)aluminum compounds used according tothe invention by reacting the abovedescribed bis(diarylphenol) ligandsof the formulae (I) or (I.a) are also lithium aluminum hydride, sodiumaluminum hydride and potassium aluminum hydride, and mixtures thereof.Moreover, mixtures of the specified compounds of the formulae (II) and(II) are also suitable for producing bis(diarylphenoxy)aluminumcompounds used according to the invention by reaction with theabovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a).

The reaction is advantageously carried out so that one of theabovedescribed bis(diarylphenol) ligands of the formulae (I) or (I.a) isbrought into contact with a compound of the formula (II) or (III). Thereaction is advantageously carried out in an inert organic solvent, suchas, for example, toluene, cyclohexane, dichloromethane, xylene,ethylbenzene, chlorobenzene, tetrahydrofuran, diethyl ether, methyltert-butyl ether, ethyl acetate, pentane, hexane, dichloroethane,dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and the like, the useof predried or anhydrous solvents being regarded as particularlyadvantageous. The reaction is usually carried out at temperatures in therange from about −100° C. to about 100° C., preferably at about −50° C.to about 50° C., particularly preferably at about −30° C. to about 30°C.

During the production of the bis(diarylphenoxy)aluminum compoundsaccording to the invention, the phenolic hydroxy groups of thebis(diarylphenol) ligands of the formulae (I) or (I.a) used react withthe compound or compounds of the formulae (II) and (III). Theoretically,each aluminum atom can react with 1 to 3 phenolic hydroxy groups. Onaccount of the steric properties or requirements of thebis(diarylphenol) ligands of the formulae (I) or (I.a) used, thisresults in the formation of higher molecular weight structures such aslinear structures or networks.

Here, the molar ratio of the bis(diarylphenol) ligands of the formulae(I) or (I.a) used to the compounds of the formula (II) and/or (III) usedis advantageously chosen such that the amount of incompletely reactedcompounds of the formulae (II) and/or (III) is as low as possible.Preferably, the specified ratio is chosen so that, after thebis(diarylphenol) ligands of the formulae (I) or (I.a) have been broughtinto contact with the compound or the compounds of the formulae (II) and(III), incompletely reacted compound of the formula (II) and/or (III) isno longer present. Taking into consideration the cost aspect, it isadvisable to keep the excess of the ligands of the formulae (I) or (I.a)used low. Particular preference is given to using bis(diarylphenol)ligands of the formulae (I) or (I.a) and the compounds of the formulae(II) and/or (III) in a molar ratio of from about 4:1 to about 1:1, veryparticularly preferably from about 3:1 to about 1.5:1 and mostpreferably in the molar ratio of about 1.5:1.

Within the scope of a preferred embodiment of the present invention, theproduction of the bis(diarylphenoxy)aluminum compounds used according tothe invention involves initially introducing, depending on thesolubility, an about 0.001 to about 1 molar solution of the chosenligand of the formula (I) or (I.a) into a suitable organic solvent, forexample toluene, at a temperature of from about −10 to about 30° C., andadding an aluminum compound of the formula (II) and/or (III), preferablyin the form of a solution, for example a solution of trimethyl- ortriethylaluminum in toluene.

The reaction between the ligands of the formula (I) or (I.a) used andthe aluminum compounds of the formulae (II) and/or (III) usually takesplace rapidly and is mostly complete after about 10 min to about 2 h,often after about 1 h, depending on the reaction conditions chosen. Whenusing more unreactive reactants, it may be advantageous to temporarilyincrease the temperature of the reaction mixture.

Depending on the reaction conditions chosen, in particular with regardto the solubility of the ligands of the formula (I) or (I.a) to bereacted and of the aluminum compound of the formula (II) and/or (III) inthe chosen solvents, the concentrations and the reaction temperatures,the bis(diarylphenoxy)aluminum compounds according to the invention areobtained in the form of a solid, a suspension or a solution in thesolvent or solvent mixture used. The bis(diarylphenoxy)aluminumcompounds used according to the invention obtained in this way can befurther used in the form obtained in each case or are separated off andfreed from the solvents used.

Isolation can take place here by methods which appear to be advantageousand are known to the person skilled in the art. Preferably, theisolation, storage and further treatment of thebis(diarylphenoxy)aluminum compounds used according to the invention arecarried out with extensive exclusion of oxygen and moisture.

To carry out the method according to the invention for producingisopulegol, the procedure advantageously involves firstly preparing asolution of the bis(diarylphenoxy)aluminum compounds used according tothe invention in a suitable solvent, as described above. The racemic ornonracemic citronellal to be cyclized is then added according to theinvention to this solution. The citronellal can here be added as it isor in the form of a solution, advantageously in one of theabovementioned suitable solvents. Within the scope of a preferredembodiment of the method according to the invention, a solution of thechosen ligand of the formulae (I) or (I.a) in toluene is firstlyprepared and then, advantageously with stirring, the chosen aluminumcompound of the formula (II) and/or (III), preferably trimethylaluminumor triethylaluminum in toluenic solution, is added.

A suitable starting material for carrying out the cyclization methodaccording to the invention is citronellal, which can be produced by anymethod. Preference is given to using citronellal which has a purity ofabout 90 to about 99.9% by weight, particularly preferably from about 95to about 99.9% by weight.

The addition of the citronellal to be cyclized advantageously takesplace at temperatures in the range from about −40° C. to about 40° C.,preferably in the range from about −20° C. to about 20° C. For this, theprepared solution of the bis(diarylphenoxy)aluminum compound usedaccording to the invention is advantageously cooled to a temperature inthis range, e.g. to a temperature in the range from −10° C. to 10° C.,and prechilled citronellal or a prechilled solution of citronellal isadded.

The addition of the citronellal or of the solution thereof can beundertaken such that either the whole amount is added at once or it isadded in portions or else continuously to the prepared catalystsolution. Suitable solvents in turn are the abovementioned solvents, inparticular toluene. Preferably, the citronellal to be cyclized is usedas it is, i.e. without the further addition of solvents. When using asolvent, the total amount of solvent (for catalyst production and forcarrying out the cyclization reaction) is advantageously chosen so thatthe volume-based ratio of citronellal to be reacted to solvent is about2:1 to about 1:20, preferably from about 1.5:1 to about 1:10.

The quantitative ratio between the citronellal to be reacted and theamount of bis(diarylphenoxy)aluminum compound employed according to theinvention used is determined by the amount of compounds of the formula(I) or (I.a) and of the formula (II) and/or (III) used for producing thesame, i.e. by the quantitative ratio of ligand used to aluminum compoundof the formula (II) and/or (III) used.

According to the invention, the amount of citronellal to be reactedrelative to the amount of aluminum compound of the formula (II) and/or(III) used is chosen such that the molar ratio is about 5:1 to about1000:1, preferably about 10:1 to about 500:1, particularly preferablyabout 50:1 to about 200:1.

Irrespective of this, the ratio between ligand of the formula (I) or(I.a) used and the aluminum compound of the formula (II) and/or (III)used can be varied within the limits specified above for producing thebis(diarylphenoxy)aluminum compound according to the invention.

The cyclization of citronellal to isopulegol generally takes placerapidly, depending on the choice of reactants and reaction conditions,and is usually largely complete after about 0.5 to about 10 h, oftenafter about 5 h. Reaction progress can be easily monitored by methodsknown per se to the person skilled in the art, for example bychromatographic, specifically gas chromatographic, methods or else HPLCmethods.

Within the scope of a preferred embodiment of the method according tothe invention, the cyclization of citronellal to isopulegol is carriedout in the presence of an auxiliary (iv), for example an acid,preferably an organic acid. By way of example, organic acids which canbe used advantageously are: acetic acid, propionic acid, benzoic acid,toluenesulfonic acid, methanesulfonic acid, preferably acetic acid. Thespecified acids are advantageously used in an amount of from about 0.5to about 10% by weight, based on the amount of citronellal to bereacted. Advantageously, they are added to the reaction mixture togetherwith the citronellal, e.g. in the form of a mixture.

In a particularly preferred embodiment, the method according to theinvention for producing isopulegol by cyclizing citronellal is carriedout in the presence of at least one auxiliary (iv) which is chosen fromcarboxylic anhydrides, aldehydes, ketones and vinyl ethers.

The auxiliaries (iv) of the specified classes of substance can in eachcase be used individually or in the form of mixtures with one another.In the case of mixtures, preference is given to using those whichconsist of compounds of one class of substance. Particular preference isgiven to using individual compounds. By using the specified compounds asdescribed below, it is generally possible to largely suppress theformation of undesired by-products.

Within the scope of a preferred embodiment, the cyclization ofcitronellal is carried out in the presence of a carboxylic anhydride ofthe formula (VI)

where the radicals R²⁰ and R^(20′) may be identical or different,preferably identical, and are a branched or unbranched C₁-C₁₂-alkylradical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, where thespecified radicals may in each case have one or more, generally 1 toabout 3, identical or different substituents chosen from the groupOR^(10e), SR^(10f)NR^(8e)R^(9e) and halogen and where R²⁰ and R^(20′)can together also form a 5- to 8-membered ring which can have one ormore ethylenic double bonds and one or more identical or differentheteroatoms chosen from the group O, S and NR^(11b) and where R^(10e),R^(10f), R^(8e), R^(9e) and R^(11b) can have the meanings given abovefor R¹¹.

Within the scope of a further preferred embodiment, the cyclization ofcitronellal is carried out in the presence of an aldehyde of the formula(VII)

where the radical R²¹ is a branched or unbranched C₁-C₁₂-alkyl radicalor C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical, where the specifiedradicals can in each case have one or more, preferably 1 to 3, identicalor different substituents chosen from the group OR^(10e),SR^(10f)NR^(8e)R^(9e) and halogen, where R^(10e), R^(10f), R^(8e) andR^(9e) can have the meanings given above for R¹¹.

Within the scope of a further preferred embodiment, cyclization ofcitronellal is carried out in the presence of a ketone of the formula(VIII)

where the radicals R²² and R²³ may in each case be identical ordifferent and are a branched or unbranched C₁-C₁₂-alkyl radical orC₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical or aC₁-C₆-alkoxycarbonyl radical, where the specified radicals can in eachcase have one or more, preferably 1 to 3, identical or differentsubstituents chosen from the group OR^(10e), SR^(10f)N^(8e)R^(9e) andhalogen, and where R²² and R²³ can together also form a 5- to 8-memberedring which can have one or more ethylenic double bonds and one or moreidentical or different heteroatoms chosen from the group O, S, NR^(11b)and where R^(10e), R^(10f), R^(8e), R^(9e) and R^(11b) can have themeanings given above for R¹¹.

As an alternative to the abovementioned carbonyl compounds, it is alsopossible to use vinyl ethers of the general formula (IX)

within the scope of the method according to the invention, where theradicals R²⁴, R²⁵, R²⁶ and R²⁷, independently of one another, may ineach case be identical or different and are a branched or unbranchedC₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical,where the specified radicals can in each case have one or more,preferably 1 to 3, identical or different substituents chosen from oxo,OR^(10e), SR^(10f)NR^(8e)R^(9e) and halogen and where R²⁵ and R²⁶ cantogether also form a 5- to 8-membered ring which can have one or moreethylenic double bonds and one or more, usually 1 or 2, identical ordifferent heteroatoms chosen from the group O, S, NR^(11b) and whereR^(10e), R^(10f), R^(8e), R^(9e) and R^(11b) can have the meanings givenabove for R¹¹.

C₁-C₁₂-Alkyl here is C₁-C₆-alkyl as described above and, moreover, forexample heptyl, octyl, nonyl, decyl, undecyl or dodecyl. In the caseswhere two alkyl radicals together form a ring, alkyl radicals are alsounderstood as meaning alkylenyl radicals. C₇-C₁₂-Aralkyl radicals andC₆-C₁₀-aryl radicals can, by way of example, have the meanings givenabove. By way of example, C₁-C₆-alkoxycarbonyl radicals which may bementioned are: methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl andisopropoxycarbonyl, preferably methoxycarbonyl and ethoxycarbonyl.

Within the scope of a preferred embodiment of the method according tothe invention, the cyclization of citronellal is carried out in thepresence of a carboxylic anhydride of the formula (VI), where theradicals R²⁰ and R^(20′) are identical and are a branched or unbranchedC₁-C₁₂-alkyl radical or C₇-C₁₂-aralkyl radical or a C₆-C₁₀-aryl radical,and where R²⁰ and R^(20′) can together also form a 5- to 8-membered ringwhich can have one or more ethylenic double bonds and one or moreidentical or different heteroatoms chosen from the group OR^(10e),SR^(10f)NR^(11b), and R^(10e), R^(10f) and R^(11b) can, independently ofone another, have the meanings given above for R¹¹.

Particular preference is given to using those carboxylic anhydrides inwhich the radicals R²⁰ and R^(20′) are identical and are a branched orunbranched C₁-C₁₂-alkyl radical or a C₆-C₁₀-aryl radical. By way ofexample, carboxylic anhydrides to be used particularly preferablyaccording to the invention are: acetic anhydride, propionic anhydride,pivalic anhydride and benzoic anhydride.

Aldehydes of the formula (VII) which can likewise be used preferablyaccording to the invention are, by way of example, acetaldehyde,propionaldehyde and chloral (trichloroacetaldehyde).

If, within the scope of a further preferred embodiment, the cyclizationof citronellal is carried out in the presence of a ketone of the formula(VIII), it is advantageous to use those with an activated, i.e.electron-deficient, carbonyl function. By way of example, mention may bemade of the following ketones which are particularly suitable for usewithin the scope of the method according to the invention:1,1,1-trifluoroacetone, 1,1,1-trifluoroacetophenone, hexafluoroacetone,methyl pyruvate and ethyl pyruvate.

Vinyl ethers of the formula (IX) which can likewise be used withpreference according to the invention are, for example: methyl vinylether, ethyl vinyl ether, isobutyl vinyl ether and 3,4-dihydro-2H-pyran.

The specified classes of compound can be used equally with good successwithin the scope of this preferred embodiment of the method according tothe invention. With regard to practical aspects such as, for example, ahigher reaction rate, the use of aldehydes and/or electron-deficientketones has proven to be advantageous.

The amount of carboxylic anhydride, aldehyde, ketone and/or vinyl etherto be used according to the invention can be varied within wide limitsand is governed by the type of substance used and the degree of purityor the presence of impurities which are as yet not more preciselyidentified. Usually, the specified compounds and mixtures thereof areused in an amount of from about 0.01 mol % to about 5 mol %, preferablyfrom about 0.1 mol % to about 2 mol %, based on the amount ofcitronellal used.

The type and manner of the procedure, for example the configuration ofreactors or the order in which individual reactants are added, are notsubject to particular requirements provided a procedure with extensiveexclusion of oxygen and water is ensured.

To carry out the method according to the invention within the scope ofthis preferred embodiment, the procedure advantageously involves firstlyproviding a solution of the bis(diarylphenoxy)aluminum compound to beused according to the invention in a suitable solvent as describedabove. Then, according to the invention, a mixture of the racemic ornonracemic citronellal to be cyclized with the chosen carboxylicanhydride, the aldehyde, the activated ketone and/or the vinyl ether ispreferably added to this solution. Alternatively thereto, it ispossible, for example, to also admix the solution of thebis(diarylphenoxy)aluminium compound to be used according to theinvention firstly with the carboxylic anhydride, of appropriate chosenin each case, the aldehyde, the ketone and/or the vinyl ether, and toafterwards add the citronellal to be cyclized.

It has proven to be advantageous to meter in the citronellal or themixture of citronellal with the chosen compound to the catalyst solutionor to the reaction mixture within a period of from about 30 min to about6 h, preferably within about 2 h to about 4 b. The citronellal can herebe added as such or in the form of a solution, advantageously in one ofthe abovementioned suitable solvents. Within the scope of an againpreferred embodiment of the method according to the invention, asolution of the chosen ligand of the formulae (I) or (I.a) in toluene isfirstly provided, and then the chosen aluminum compound of the formula(II) and/or (III), preferably trimethylaluminum or triethylaluminum intoluenic solution is added, expediently with stirring.

The addition of the citronellal to be cyclized or the mixture ofcitronellal with the chosen carboxylic anhydride, aldehyde, activatedketone and/or the vinyl ether takes place within the scope of thisembodiment advantageously at temperatures in the range from about −40°C. to about 40° C., preferably in the range from about −20° C. to about20° C. For this, the prepared solution or suspension of thebis(diarylphenoxy)aluminum compound according to the invention isadvantageously cooled to a temperature within this range, e.g. to atemperature in the range from −10° C. to 10° C., and the other reactantsare added in precooled form.

The addition of the mixture of citronellal and the chosen furthercompound can be undertaken so that either the total amount ofcitronellal is added in one go or it is added in portions orcontinuously to the prepared catalyst solution. Suitable solvents are inturn preferably the abovementioned solvents, in particular toluene.Preference is given to using the citronellal to be cyclized in the formof a mixture with the chosen carboxylic anhydride, aldehyde, activatedketone and/or vinyl ether without the further addition of solvents. Whenusing a solvent, the total amount of solvent is advantageously chosen sothat the volume-based ratio of citronellal to be reacted to the solventis about 1:1 to about 1:20, preferably from about 1:1 to about 1:10.

It has been found that some of the catalyst complex is usuallydeactivated during the reaction. This is attributed, inter alia, toligand exchange processes between the bis(diarylphenol) ligands of theformula used in each case of the bis(diarylphenoxy)aluminum compoundsused and the isopulegol which forms as a result of cyclization. Thedeactivated form of the catalyst is, depending on the choice of solventused, soluble in the reaction mixture, usually in contrast to the activepolymeric catalyst.

In one preferred embodiment, the deactivated part of the catalyst can beseparated off together with the other reaction mixture by simplephysical separation methods (e.g. by filtering off or centrifuging thecatalyst which is still active). The retained, still active part of thecatalyst can, if desired, be supplemented with a flesh catalyst and bereused without appreciable loss in activity, preferably within the scopeof a further cyclization reaction according to the invention ofcitronellal to isopulegol.

Alternatively, the amount of catalyst used can be chosen so that thetotal catalyst complex used is deactivated and thus soluble in thecourse of or at the end of the cyclization reaction according to theinvention, something which is recognizable from a clear reactionmixture. Here, it is advantageously notable that in this case, onaccount of the abovedescribed ligand exchange processes, thebis(diarylphenol) ligand of the formula (I) used in each case isreleased without separate hydrolysis being carried out.

Surprisingly, it has been found that isopulegol can be distilled offfrom the aluminum-containing reaction products of the cyclization ofcitronellal without prior hydrolysis of the bis(diarylphenoxy)aluminumcompounds used in each case as catalyst (if appropriate followingdistillative removal of a solvent used and/or additionally usedauxiliaries) in high purities. As a rule, no recognizable undesired ortroublesome by-products form in the distillation bottom. In a specificembodiment, a suitable, inert, high-boiling solvent is added before orduring the distillative separation in step a). A solution of the ligandof the formula (I) in the heated high-boiling component used in eachcase is then obtained in the distillation bottom.

As already mentioned, the method according to the invention is equallysuitable for cyclizing racemic and nonracemic, i.e. optically active,citronellal to give racemic and nonracemic isopulegol.

In a preferred embodiment, the method according to the invention thusserves for producing optically active isopulegol of the formula (IV.a)

by cyclization of active citronellal of the formula (V.a)

where (*) in each case refers to an asymmetric carbon atom.

The method according to the invention serves in particular for producingL-(−)-isopulegol by cyclization of D-(+)-citronellal.

The racemic or nonracemic isopulegol produced in this way is a valuableintermediate for producing racemic or nonracemic menthol, one of themost significant fragrances or aromas worldwide. Menthol can be obtainedfrom isopulegol by methods of hydrogenation known per se to the personskilled in the art, specifically catalytic hydrogenation over suitabletransition metal catalysts, as described, for example, in Pickard etal., J. Chem. Soc. 1920, 1253; Ohloff et al., Chem. Ber. 1962, 95, 1400;Pavia et al., Bull. Soc. Chim. Fr. 1981, 24, Otsuka et al., Synthesis1991, 665 or in EP 1 053 974 A. Here, if the chosen reaction conditionsare suitable, the relative or absolute configuration of the isopulegolused is largely retained, and in many cases is completely retained.

The present invention therefore further provides a method of producingmenthol comprising the steps:

A) production of isopulegol of the formula (IV) by a method according tothe invention

B) hydrogenation of the ethylenic double bond of the isopulegol obtainedin this way.

In a preferred embodiment, this method serves for producing opticallyactive menthol, specifically for producing L-(−)-menthol from opticallyactive L-(−)-isopulegol.

With regard to the preferred embodiments of the method according to theinvention for producing isopulegol, reference is made to theabovementioned preferences in their entirety.

The examples below serve to illustrate the present invention withouthaving any limiting character.

EXAMPLE 1 Process for recovering1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3)

Gas chromatographic analyses (GC) were carried out according to thefollowing method: 50 m CP-WAX, ID.: 0.32 mm, FD.: 1.2 ym; 80° C., 3°C./min −200° C., 15° C./min to 250° C.; t_(R) (phenylcyclohexane): 30.7,t_(R) (isopulegol): 26.3; t_(R) (citronellal): 21.8.

The following HPLC method was used: CC250/4 Nucleodur C18 Gravity, 5 ym;C: water—0.05% H₃PO₄; D: acetonitrile 20:80; exit: 93 bar, 25° C.; t_(R)(phenylcyclohexane): 10.5; t_(R) (isopulegol): 3.3; t_(R) (ligand(Ia₂-3)): 14.0. Concentrations of the reaction products obtained in thedistillation bottom and in the mother liquor (in each case in % byweight) were determined analytically by GC and HPLC using an internalstandard.

1.a) Cyclization of Citronellal

1,1-Bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3) (461 g,0.785 mmol) in anhydrous toluene (7.2 l) was initially introduced into ajacketed glass reactor with stirrer. A solution of triethylaluminum intoluene (445 ml, 400 mmol, 12% AlEt₃ in toluene) was added to the clearsolution of the ligand at room temperature. The solution was stirred for1 h at 25° C. The resulting catalyst suspension was cooled to 0° C. andadmixed over a period of 3 h with a mixture of citronellal (6697 g, 43mol) and methyl pyruvate (33.6 g, 329 mmol). When addition was complete,the reaction mixture was after-stirred for 3 h at 0° C. and for afurther 2 h at 110° C. Toluene was separated off under reduced pressure.An isopulegol crude product was then separated off by distillation astop product with the addition of phenylcyclohexane (2770 g). 3584 g of abottom product were obtained.

1.b) Isolation of the Ligand (Ia₂-3)

3564 g of the bottom product from the cyclization of citronellal in thepresence of a (bis(diarylphenoxy))aluminum catalyst comprisingphenylcyclohexane (69.9% by weight), isopulegol (3.05% by weight),citronellal (0.16% by weight) and citronellol (3.05% by weight) wereinitially introduced into a jacketed reactor with stirrer and refluxcondenser at a temperature of 90° C. 1792 g of a heated 2% strengthaqueous NaOH solution were added to the heated solution. After stirringfor one hour at 90° C., 1777 g of the aqueous phase were separated offfrom the organic phase. The remaining water from the organic phase wasdistilled off at 120° C. and 10 mbar. The hydrolyzed bottom product wascooled to 25° C. over the course of 12 hours. The resulting suspensionof the ligand of the formula (Ia₂-3) was filtered and the ligand of theformula (Ia₂-3) obtained in this way was freed from volatileconstituents at 3 mbar and 95° C. The ligand of the formula (Ia₂-3) wasisolated as white solid with a yield of 282 g and a purity of 95%.According to HPLC analysis, the mother liquor (3130 g) comprisedphenylcyclohexane (72.3% by weight), isopulegol (6.8% by weight) andligand of the formula (Ia₂-3) (4.9% by weight). This demonstrates thatthe ligands used according to the invention are suitable in anadvantageous manner for a continuous work-up. By contrast, when usingthe ligands described in EP-A 1 225 163, separation of the phases is notensured in every case since these have a greater tendency to form stableemulsions.

EXAMPLE 2 Continuous process for recovering1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3)

Analysis

Gas chromatographic analyses were carried out according to the followingmethod: 50 m CP-WAX, ID).: 0.32 mm, FD.: 1.2 ym; 80° C., 3° C./min −200°C., 15° C./min to 250° C.; t_(R) (citronellal): 20.7; t_(R)(isopulegol): 24.7; t_(R) (phenylcyclohexane): 29.3; t_(R)(citronellol): 31.7; t_(R) (citronellol citronellate): 48.2; t_(R)(isopulegyl citronellate): 49.5.

2.a) Cyclization of Citronellal During Continuous Work-Up

A solution of triethylaluminum in toluene (15% strength, 85 ml, 0.096mol) is added at 20° C. over the course of about 10 min to a clearsolution of 1,1-bis(2,6-diphenylphenol)-1-trifluoromethylethane (Ia₂-3)(114 g, 0.195 mol) in toluene (anhydrous, 1800 g) in a jacketed glassreactor with stirrer. The solution is then stirred at 20° C. for 1 h.The resulting catalyst suspension is transferred to a further jacketedglass reactor with stirrer, cooled to 0° C. and admixed over a period of3 h with a mixture of D-citronellal (1620 g, 10.3 mol) and methylpyruvate (8.1 g). When addition is complete, the reaction solution isstirred at 0° C. until a content of <10 GC area % of D-citronellal isreached, warmed to 10° C. and stirred for a further 2 h at thistemperature. Subsequently, the reaction solution is firstly transferredto a buffer container.

The reaction solution is passed to a plate column (15 plates, DN 50)continuously at a feed rate of 300 g/h. Toluene is removed from thecolumn at a head pressure of about 100 mbar at a side take-off at the10th plate in the enriching section, where the bottom temperature isabout 120° C. and the temperature of the side take-off and of the top ofthe column are 45° C. The low-boiling components are eliminated from thereaction solution at the top of this column.

A discharge of the bottom product from the plate column is fedcontinuously (120 to 140 g/h) into the center of a packed column (DN50×120 cm, laboratory fabric packing, Sulzer DX). With the continuousaddition of phenylcyclohexane (70 to 90 g/h) into the bottom of thispacked column, L-isopulegol is distilled off as top product at a bottomtemperature of 110° C. and a head pressure of 10 mbar. L-isopulegol isisolated in a yield of 1625 g and in a purity of 93%.

2.b) Isolation of the Ligand (Ia₂-3) During Continuous Procedure

A discharge of the distillation bottom of the packed column is fedcontinuously (100 to 120 g/h) to a mixer-settler apparatus heated to 95°C. and consisting of two cascaded 250 ml stirred containers and a 150 mlphase separator. In the first 250 ml stirred container, the discharge ofthe distillation bottom of the packed column is admixed continuouslywith a feed of 2% strength sodium hydroxide solution (50 to 60 g/h). Adischarge (150 to 180 g/h) of the mixed phase from the first stirredcontainer is transferred to the 150 ml phase separator via the other 250ml stirred container. In the phase separator, the continuous separationof the phases takes place at a temperature of from 90 to 95° C. Theheight of the phase separation layer is regulated here with the help ofconductivity measurements.

The discharge of the organic phase from the phase separator is collectedcontinuously (100 to 120 g/h) in a further stirred container heated to40 to 50° C. and left to crystallize prior to isolation of the ligand(Ia₂-3). A discharge of the aqueous phase from the phase separator iscontinuously pumped off.

The crystallized ligand (Ia₂-3) is filtered batchwise through a pressurefilter at a nitrogen pressure of 4 bar. The filtercake is then washedwith phenylcyclohexane. The washed ligand (106 g; HPLC % by weight:ligand 77%; phenylcyclohexane 22%) is dissolved in toluene and furtherused for preparing the catalyst in step 2.a). The filtrate (919 g; % byweight according to GC; phenylcyclohexane 66%; L-isopulegol 5%;citronellol 6.1%; isopulegyl citronellate 4.3%; citronellyl citronellate3.6%; % by weight according to HPLC: ligand 3.1%) is returned to thepacked column described under 2.b).

The invention claimed is:
 1. A method for working up analuminum-containing reaction product from the production of isopulegolby cyclizing citronellal, comprising i) isopulegol; and ii) at least oneligand of the formula (I),

wherein Ar¹, Ar², Ar³, and Ar⁴ are, independently of one another, aC₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein saidC₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionallysubstituted with up to 7 identical or different substituents selectedfrom the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5a)R^(6a)R^(7a), Koptionally substituted C₆-C₁₀-aryl, NR^(8a)R^(9a), SR^(10a), and NO₂;R¹, R², R³, and R⁴ are, independently of one another, hydrogen,C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl,halogen, SiR^(5b)R^(6b)R^(7b), optionally substituted C₆-C₁₀-aryl,NR^(8b)R^(9b), SR^(10b), or NO₂, wherein R¹ or R² and/or R³ or R⁴together with A optionally define an aromatic or nonaromatic cycle; andA is a straight-chain or branched and/or cyclic hydrocarbon radicalhaving up to 25 carbon atoms, wherein said straight-chain or branchedand/or cyclic hydrocarbon radical is optionally saturated or mono- orpolyunsaturated and/or partially aromatic, optionally has one or moreidentical or different heteroatoms selected from the group consisting ofO, S, and NR¹¹ and/or one or more identical or different functionalgroups selected from the group consisting of C(O), S(O), and S(O)₂, andis optionally substituted with one or more identical or differentsubstituents selected from the group consisting of C₁-C₆-alkyl,C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl,halogen, —SiR^(5c)R^(7c), optionally substituted C₆-C₁₀-aryl, optionallysubstituted C₂-C₁₀-hetaryl, NR^(8c)R^(9c), SR^(10c), NO₂, C₁-C₁₂-acyl,and C₁-C₁₀-carboxyl, or is a C₆-C₁₅-aryl radical or a C₂-C₁₅-heteroarylradical, wherein said C₆-C₁₅-aryl radical and said C₂-C₁₅-heteroarylradical is optionally substituted with up to 5 substituents selectedfrom the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5d)R^(6d), R^(7d),optionally substituted C₆-C₁₀-aryl, NR^(8d)R^(9d), SR^(10d), and NO₂, oris a functional group or a heteroatom selected from the group consistingof —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—, —S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—,and —Si(R¹²R¹³); wherein R^(5a), R^(6a), R^(7a), R^(8a), R^(9a),R^(10a), R^(5b), R^(6b), R^(7b), R^(8b), R^(9b), R^(10b), R^(5c),R^(6c), R^(7c), R^(8c), R^(9c), R^(10c), R^(5d), R^(6d), R^(7d), R^(8d),R^(9d), R^(10d), R¹¹, R¹², and R¹³ are in each case independently of oneanother selected from the group consisting of C₁-C₆-alkyl,C₇-C₁₂-aralkyl, and/or optionally substituted C₆-C₁₀-aryl, and whereinR^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and R^(9c), R^(8d) andR^(9d), independently of one another, in each case optionally define acyclic hydrocarbon radical having 2 to 8 carbon atoms which optionallyhas one or more identical or different heteroatoms selected from thegroup consisting of O, S, and NR^(11a), wherein R^(11a) is as defined asR¹¹; in free and/or complex-bound form; said method comprising: a)subjecting the reaction product to distillative separation to obtain anisopulegol-enriched top product and an isopulegol-depleted bottomproduct; b) bringing said isopulegol-depleted bottom product into closecontact with an aqueous base to give an aluminum-containing aqueousphase and an organic phase comprising the majority of the ligands offormula (I); c) separating off the ligands of formula (I) from saidorganic phase.
 2. The method of claim 1, wherein said ligand of formula(I) is separated off from the organic phase in c) by crystallization. 3.The method of claim 1, wherein said aluminum-containing reaction productfrom the production of isopulegol by cyclizing citronellal additionallycomprises a lower-boiling solvent (iii).
 4. The method of claim 1,wherein said aluminum-containing reaction product from the production ofisopulegol by cyclizing citronellal additionally comprises an auxiliary(iv).
 5. The method of claim 4, where said auxiliary (iv) is selectedfrom the group consisting of organic acids, carboxylic anhydrides,aldehydes, ketones, and vinyl ethers.
 6. The method of claim 1, wherein,prior to the distillative separation in a), any solvent present and/orauxiliaries from the cyclization are first separated off from saidreaction product.
 7. The method of claim 1, wherein, prior to and/orduring the distillative separation in a), said reaction product isadmixed with a solvent whose boiling point, under the distillationconditions, is at least 10° C. higher than the boiling point ofisopulegol.
 8. The method of claim 1, wherein at least one of a), b),and c) is operated continuously.
 9. The method of claim 7, wherein theaddition of the higher-boiling solvent takes place during a).
 10. Themethod of claim 1, wherein a heated discharge of the bottom product froma) is brought into close contact with a heated aqueous base and then themajority of the ligand is isolated from the organic phase.
 11. Themethod of claim 1, wherein the ligand of formula (I) is abis(diarylphenol) ligand of formula (I.a)


12. A method for producing isopulegol of formula (IV)

comprising a) cyclizing citronellal of formula (V)

in the presence of a catalyst obtained by reacting a bis(diarylphenol)ligand of formula (I)

wherein Ar¹, Ar², Ar³, and Ar⁴ are, independently of one another, aC₆-C₁₅-aryl radical or a C₂-C₁₅-heteroaryl radical, wherein saidC₆-C₁₅-aryl radical and said C₂-C₁₅-heteroaryl radical is optionallysubstituted with up to 7 identical or different substituents selectedfrom the group consisting of C₁-C₆-alkyl, C₁-C₆-perfluoroalkyl,C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen, SiR^(5a)R^(6a)R^(7a), optionallysubstituted C₆-C₁₀-aryl, NR^(8a)R^(9a), SR^(10a), and NO₂; R¹, R², R³,and R⁴ are, independently of one another, hydrogen, C₁-C₆-alkyl,C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen,SiR^(5b)R^(6b)R^(6b), optionally substituted C₆-C₁₀-aryl, NR^(8b)R^(9b),SR^(10b), or NO₂, wherein R¹ or R² and/or R³ or R⁴ together with Aoptionally define an aromatic or nonaromatic cycle; and A is astraight-chain or branched and/or cyclic hydrocarbon radical having upto 25 carbon atoms, wherein said straight-chain or branched and/orcyclic hydrocarbon radical is optionally saturated or mono- orpolyunsaturated and/or partially aromatic, optionally has one or moreidentical or different heteroatoms selected from the group consisting ofO, S, and NR¹¹ and/or one or more identical or different functionalgroups selected from the group consisting of C(O), S(O), and S(O)₂, andis optionally substituted with one or more identical or differentsubstituents selected from the group consisting of C₁-C₆-alkyl,C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₁-C₁₀-acyloxy, C₇-C₁₂-aralkyl,halogen, —SiR^(5c)R^(6c)R^(7c), optionally substituted C₆-C₁₀-aryl,optionally substituted C₂-C₁₀-hetaryl, NR^(8c)R^(9c), SR^(10c), NO₂,C₁-C₁₂-acyl, and C₁-C₁₀-carboxyl, or is a C₆-C₁₅-aryl radical or aC₂-C₁₅-heteroaryl radical, wherein said C₆-C₁₅-aryl radical and saidC₂-C₁₅-heteroaryl radical is optionally substituted with up to 5substituents selected from the group consisting of C₁-C₆-alkyl,C₁-C₆-perfluoroalkyl, C₁-C₆-alkoxy, C₇-C₁₂-aralkyl, halogen,SiR^(5d)R^(6d)R^(7d), optionally substituted C₆-C₁₀-aryl, NR^(8d)R^(9d),SR^(10d), and NO₂, or is a functional group or a heteroatom selectedfrom the group consisting of —O—, —S—, —N(R¹¹)—, —S(O)—, —C(O)—,—S(O)₂—, —P(R¹¹)—, —(R¹¹)P(O)—, and —Si(R¹²R¹³); wherein R^(5a), R^(6a),R^(7a), R^(8a), R^(9a), R^(10a), R^(5b), R^(6b), R^(7b), R^(8b), R^(9b),R^(10b), R^(5a), R^(6c), R^(7c), R^(8c), R^(9c), R^(10c), R^(5d),R^(6d), R^(7d), R^(8d), R^(9d), R^(10d), R¹¹, R¹², and R¹³ are in eachcase independently of one another selected from the group consisting ofC₁-C₆-alkyl, C₇-C₁₂-aralkyl, and/or optionally substituted C₆-C₁₀-aryl,and wherein R^(8a) and R^(9a), R^(8b) and R^(9b), R^(8c) and R^(9c),R^(8d) and R^(9d), independently of one another, in each case optionallydefine a cyclic hydrocarbon radical having 2 to 8 carbon atoms whichoptionally has one or more identical or different heteroatoms selectedfrom the group consisting of O, S, and NR^(11a), wherein R^(11a) is asdefined as R¹¹; with an aluminum compound of formula (II):(R¹⁴)_(3-p)AlH_(p)  (II) wherein Al is aluminum; R¹⁴ is a branched orunbranched alkyl radical having up to 5 carbon atoms; and p is 0 or aninteger from 1 to 3; and/or with an aluminum compound of the formula(III):MAlH₄  (III) wherein Al is aluminum; and M is lithium, sodium, orpotassium; and β) recovering the bis(diarylphenol) ligand of formula (I)after the reaction has taken place by a) subjecting the reaction productobtained in α) to distillative separation to obtain anisopulegol-enriched top product and an isopulegol-depleted bottomproduct; b) bringing the isopulegol-depleted bottom product into closecontact with an aqueous base to give an aluminum-containing aqueousphase and an organic phase comprising the majority of the ligands offormula (I); and c) separating off the ligand of formula (I) from theorganic phase.
 13. The method of claim 12, wherein the ligand of formula(I) is separated off from the organic phase in c) by crystallization.14. The method of claim 12, wherein said aluminum compound of formula(II) is selected from the group consisting of trimethylaluminum andtriethylaluminum.
 15. The method of claim 12, wherein the isopulegol offormula (IV) produced is optically active isopulegol of formula (IV.a)

wherein the citronellal of formula (V) cyclized in α) is opticallyactive citronellal of formula (V.a)

wherein (*), in each instance, indicates an asymmetric carbon atom. 16.The method of claim 15, wherein the optically active isopulegol offormula (IV.a) is L-(−)-ispulegol and the optically active citronellalof formula (V.a) is D-(+)-citronellal.
 17. A method for producingmenthol, comprising: A) producing isopulegol of formula (IV) accordingto the method of claim 12; and B) hydrogenating the ethylenic doublebond of the isopulegol of formula (IV) obtained in A).
 18. A method forproducing optically active methol, comprising: A) producing opticallyactive isopulegol of formula (IV.a) according to the method of claim 15;and B) hydrogenating the ethylenic double bond of the optically activeisopulegol of formula (IV.a) obtained in A).
 19. A method for producingL-(−)-menthol, comprising: A) producing L-(−)-isopulegol according tothe method of claim 16; and B) hydrogenating the ethylenic double bondof the L-(−)-isopulegol obtained in A).